4-(Dimethylamino)pyridine (DMAP) as an Acid-Modulated Donor Ligand for PAH Dearomatization

Abstract

The dearomatization of naphthalene and anthracene is explored by their η 2 coordination to {TpMo-(NO)(MeIm)} and {TpMo(NO)(DMAP)} (where Tp = hydridotris(pyrazolyl)borate, MeIm = 1-methylimidazole, and DMAP = 4-(dimethylamino)pyridine). The DMAP and MeIm complexes have nearly identical redox properties and abilities to bind these polycyclic aromatic hydrocarbons (PAHs), but unlike MeIm, the DMAP ligand can be protonated at N while remaining bound to the metal. This action enhances the πacidic properties of DMAP, resulting in greater stability of the molybdenum toward oxidation by acid. Utilizing this feature of the DMAP ligand, several new 1,2-dihydronaphthalenes and 1,2-dihydroanthracenes were prepared. Furthermore, it was found that acetals and Michael acceptors could function as electrophiles for the PAHs using the DMAP system, resulting in several new mono-and 1,4-dialkylated products.

Molybdenum-Promoted Dearomatization of Pyridines

“…Initial investigations of the reactivity of the dihapto-coordinate pyridine complexes focused on protonation. These molybdenum complexes are highly susceptible to acid oxidation, and thus strong acid and low temperatures are required to ensure complete and irreversible protonation at nitrogen. Once protonated in this manner, the Mo­(I)/Mo(0) reduction potential is shifted dramatically positive (vide infra), minimizing the chance of metal oxidation.…”

“…CV (MeCN) E p,a = −0.42 V (NHE). 1 Synthesis of TpMo(NO)(DMAP)(3,4-η 2 -N-methylpyridin-2(1H)-one) (29). Compound 24 (1.0 g, 0.0013 mol), DCM (5 mL), and propylamine (1.2 g, 0.020 mol) were added to a 4 dram vial containing a stir pea.…”

Michael–Michael Ring-Closure Reactions for a Dihapto-Coordinated Naphthalene Complex of Molybdenum

“…In order for the desired sequence to be successful, the molybdenum complex would have to sufficiently activate the naphthalene for the initial conjugate addition reaction at C4 (Scheme ) and stabilize the 4H-naphthalenium product long enough for the second enolate to close at C3. The complex MoTp­(NO)­(DMAP)­(η 2 -naphthalene) ( 1 ) was prepared on a 10 g scale according to literature methods as a 3:1 mixture of coordination diastereomers by reducing MoTp­(NO)­(DMAP)­(I) with magnesium or sodium in the presence of naphthalene. , …”

“…The addition of either LiOTf or BF 3 ·OEt 2 yielded only free naphthalene after treatment with an oxidant (I 2 or air). However, treating a propionitrile solution of 1 and EVK (−60 °C) with trimethylsilyltriflate (TMSOTf) generated a vivid red solution, indicative of a π-allyl complex of {MoTp­(NO)-(DMAP)} . After it was stirred for an additional hour, the solution was loaded onto an alumina column, and compound 2 was isolated as a yellow solid (45%).…”

“…We have a long-standing interest in promoting novel organic reactions of aromatic molecules through their dihapto-coordination, − and we recently investigated the ability of the complex {MoTp­(NO)­(DMAP)} (DMAP = 4-(dimethylamino)­pyridine; Tp = tris­(pyrazolyl)­borate) to dearomatize naphthalene via a 1,4-tandem addition reaction sequence (Scheme ). , We questioned whether this highly pi-basic metal complex was sufficiently electron-donating to promote a MIMIRC sequence, using this common aromatic hydrocarbon as the initial nucleophile (Scheme ). Herein we describe our attempts to effect such reactions, both where the Michael acceptors are the same (R′ = R″) and when they are different (R′ ≠ R″), using a Lewis acid to initiate the process.…”